Fragment output R, RG, RGBA 8-bit formats.

Fragment output R, RG, RGBA 8-bit formats.

Relevant dEQP tests:
dEQP-GLES3.functional.fragment_out.basic.int.r8i*
dEQP-GLES3.functional.fragment_out.basic.uint.r8ui*
dEQP-GLES3.functional.fragment_out.basic.int.rg8i*
dEQP-GLES3.functional.fragment_out.basic.uint.rg8ui*
dEQP-GLES3.functional.fragment_out.basic.int.rgba8i*
dEQP-GLES3.functional.fragment_out.basic.uint.rgba8ui*

Change-Id: I2cfb79339830e2ae22ac902ce97b6c81559e0f53
Reviewed-on: https://swiftshader-review.googlesource.com/5630
Tested-by: Meng-Lin Wu <marleymoo@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>